High Memory Usage on VPS servers is 80%

I am new to Cpannel usage and I am having 8GB RAM VPS server and 2GB RAM VPS server ,there are 6 websites running on 8GB server and its RMA usage is almost 80% and this server has 24 CPU .Websites are based on PHP and running on Mysql .I am trying to find the culprit of this high Memory Usage,Can any one help me what functions i need to use in cpannel to find out the culprit process